Regulation of muscle contraction will be studied at the molecular level in vertebrate and in invertebrate muscles. Interaction of calcium with the regulatory components of muscle will be analyzed employing a comparative approach. Two control systems are known in muscles. Regulation by troponin alters the availability of actin sites; regulation by light chains of myosin blocks myosin sites; both control systems maintain the resting state in the absence of calcium by preventing crosslink formation between actin and myosin. The proposed research is focused on defining better the mechanism of the subunit regulation of myosin. The domains on the light chains and on the heavy chains necessary for interaction, calcium binding and calcium regulation, will be further characterized by studying the hybrids between the subunits of various regulated and non-regulated myosins, by looking for common antigenic regions, by looking for calcium dependent intrinsic fluorescence changes of tryptophane residues, by attempting to crystallize regulatory light chains to obtain their three-dimensional structures, by comparing the structure of intact striated scallop muscles with scallop muscles from which regulatory light chains have been removed, and by analyzing the complexes formed between scallop heavy meromyosin and subfragment-1 and actin without and with regulatory light chains. We will look for possible defects in troponin components, both in molluscan muscles and in mutant mice. Anchoring of actin to Z-lines, dense bodies and membrane structures will be studied by attempting to isolate those components that interact with actin from different animals.